Hook these platforms restart code into the new restart hook rather
than using arch_reset().

In doing so, we split out the ixdp2351 restart code into its own
platform file.
Acked-by: NLennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>